A new U.S. political survey shows Donald Trump is increasing his lead in the crowded race for the 2016 Republican presidential nomination.

Thursday's Quinnipiac University poll showed the brash billionaire real estate mogul with 28 percent support among Republican voters across the country. He easily topped his nearest challenger in the 17-candidate field — former neurosurgeon Ben Carson, another political novice, who had 12 percent support across the country.

Former Florida Governor Jeb Bush, the son and brother of two former U.S. presidents; U.S. Senator Ted Cruz of Texas and U.S. Senator Marco Rubio of Florida each had 7 percent support among the 666 Republicans surveyed.

A month ago, the same poll pegged Trump's support at 20 percent. His current margin is the largest the Quinnipiac pollsters have recorded between first- and second-place candidates during multiple surveys ahead of the next year's primary elections and caucuses, which begin in February.

The flamboyant Trump's verbal taunts at his opponents and journalists have drawn the support of some Republicans who are tired of political wrangling in Washington. But his outbursts have dismayed other Republicans, many of whom said there was no way they'd support him.

Clinton leads Democrats

Former Secretary of State Hillary Clinton continues to lead the Democratic presidential contenders. Quinnipiac said her lead was dwindling, however, amid doubts about her honesty related to a private email account she used to handle government business when she was the country's top diplomat.

The survey of 647 Democrats showed Clinton with 45 percent support, down from 55 percent a month ago. She was followed by Vermont Senator Bernie Sanders with 22 percent and Vice President Joe Biden with 18 percent. Biden has not entered the presidential race but is weighing a late run at the nomination.

Looking beyond the party races toward the general election in November 2016, the Quinnipiac poll asked voters to predict the outcome of hypothetical contests. The results indicated Clinton would defeat any of the current Republican challengers, but that Biden would fare even better if he were the Democratic nominee.

The poll showed that many voters were unhappy with their prospective choices in the national election. When asked for the first word that came to mind about Clinton, the most frequent choice was "liar." In Trump's case, those who were surveyed chose "arrogant" to describe him. For Bush, most voters said "Bush" was their first mental association — possibly a reference to the continuing unpopularity of his brother, former President George W. Bush.

President Barack Obama is constitutionally barred from running for a third term. Following next year's election, the nation's 45th president will take office in January 2017.